Soil Suction Research Articles

Understanding and mitigating settlement in gypseous soils: A comprehensive study on the role of saturation and soil stabilization techniques

This study examines the influence of saturation levels and soil additives on the load-settlement properties of gypseous soils, a significant geotechnical concern in regions like Iraq. The study analyzes the deficiencies of conventional oedometer testing, which often fail to adequately represent the true unsaturated conditions prevalent in these soils. The research investigates the impact of varying matric suction (Ψ) values on the compressibility of untreated, cement-treated, and CKD-treated gypseous soils. A modified oedometer apparatus, engineered to control and quantify matric suction, was used to replicate diverse moisture conditions. The experimental program investigated soils with varying gypsum contents (8.8%, 12.66%, and 30%), reflecting the diversity of gypseous soils seen in the field. The findings demonstrate that increased saturation levels significantly enhance settling in gypseous soils. As matric suction increases, settlement is markedly reduced due to elevated soil suction pressure and enhanced effective stress. The findings underscore the importance of unsaturated conditions in improving the load-bearing capacity of these soils. The study illustrates the advantageous impacts of cement and CKD treatments in reducing settling. The effectiveness of these methods is particularly apparent in partially saturated environments, emphasizing the need of including matric suction into design considerations. This research improves understanding of the complex behavior of unsaturated gypseous soils. The findings have practical importance for geotechnical engineering, including foundation design, ground improvement methods, and the development of effective solutions for settlement issues in areas with high gypsum concentrations.

Three-dimensional numerical modeling of soil-roots system based on X-ray computed tomography: Hydraulic effects study

Vegetation roots enhance soil stability by influencing saturation and pore structure, playing a pivotal role in stabilizing slopes, reducing erosion, and enhancing soil structure. However, current research on the hydraulic effects of roots on soil remains relatively limited. The micro-mechanisms of vegetation's impact on soil and the macro-level connections are not yet fully understood, which poses a challenge to the modeling of root-soil system. This study develops a three-dimensional (3D) finite element model of root-soil composites based on root computed tomography (CT) images and experimental results. Four different groups are modeled, including the rootless group, and those with Festuca arundinacea (FA) roots at various growth stages. The simulation results show that the saturation in the shallow layers significantly decreases in root-soil composite groups, and the rhizosphere water content is lower than that away from the roots, resulting in a net water flux toward the roots. The influence range of roots on suction is gradually amplified with increasing root growth process and root water uptake time. Higher levels of root development result in a stronger overall water uptake effect, leading to a more pronounced decrease in saturation. Closer proximity to the surface roots results in a more rapid increase in soil suction. Compared with one-dimensional root water uptake models, this model considers the effects of spatial heterogeneity of root structures on soil, which provides a comprehensive modeling basis for studying the effect of root system on soil.

Estimating soil water retention curves over the entire saturation range: A thermal conductivity-based approach

The relationship between soil water content (θ) and suction (h, referring to the absolute value of pressure head), is described by the soil water retention curve (SWRC). Our earlier research (Fu et al. (2021, 2023a) [J. Hydrol.127171]; [J. Hydrol.129898]) recognized underlying correlations between SWRCs and soil thermal conductivity (λ) versus θ curves, and developed methodologies to ascertain the parameters of the van Genuchten (vG) equation using λ(θ) measurements, described by the Ghanbarian & Daigle (GD) equation, and basic soil characteristics. Limitations intrinsic to the van Genuchten equation restrict the GD-vG approach to generate precise estimates only in the wet and medium suction range, specifically h ranging from 0 to 150 mH2O. The validity of these approaches in the dry region remains uncertain. In this study, we associated the Peters-Durner-Iden (PDI) model parameters to those of the GD model. An initial examination was performed on the linearization processes needed to derive the hydraulic continuity water content (θhc) from the capillary water component as characterized by the PDI model and to choose the suction at oven dryness (h0) based on PDI model performance. Subsequently, two piecewise functions and two pedo-transfer functions were formulated to compute the PDI model parameters utilizing soil porosity, particle size distribution, and GD parameters, based on a calibration dataset comprising 25 different soils. The new GD-PDI approach was subsequently assessed with six independent soils and juxtaposed with the previous GD-vG approach. The GD-PDI approach outperformed the GD-vG approach, particularly within the dry range.

Effects of Wooden Embers Cover on thermo-hydrological response of silty volcanic cover and implications to post-wildfire slope stability

Wildfires striking vegetated hillslopes appear to increase the hazard towards rainfall-induced landslides. One mechanism little investigated in the literature consists in the formation of Wooden Embers Cover (WEC) following the wildfire. This layer has very peculiar thermohydraulic properties and may affect the interaction between the atmosphere and the subsoil. The paper presents an experiment conducted in an outdoor lysimeter filled with pyroclastic silt (SILT) up to 75 cm covered with 5 cm of WEC. Water storage in the SILT layer, soil water content, suction, and temperature were recorded for several years, initially under bare (no-WEC) condition (4 years), then vegetated (no-WEC) condition (5 years) and, finally, with a WEC placed on the top of the SILT (SILT+WEC condition; 3 years). The hydrological effect of the WEC was assessed by comparing the response of the SILT+WEC with the SILT under bare or vegetated conditions. The WEC reduces water losses by evaporation, thus increasing the average water content in the underlying SILT, an effect that is detrimental to slope stability. To discriminate whether the barrier effect was associated with the lower thermal or hydraulic conductivity of the WEC, a numerical simulation was carried out by considering the case of a WEC with its real thermal and hydraulic properties and the case of a fictitious top layer placed on the top of the SILT having the same hydraulic properties of the WEC but the thermal properties of the SILT. It is concluded that the barrier effect of the WEC is mainly associated with its hydraulic properties, i.e. the WEC acts as a capillary barrier. To demonstrate the practical implications of this findings, a case study of rainfall-induced landslide has been reanalysed by simulating the presence of a WEC layer having the same thermohydraulic properties as the material characterised in this study. It is shown that a WEC can substantially reduce the severity of the triggering rainfall event, thus increasing the vulnerability of the slope to rainfall-induced failure.

Effect of soil type on tipping point hydrological requirements for Axonopus compressus grass under extreme drought stress

Critical soil suctions (threshold, tipping point, and permanent wilting) corresponding to initial drought response, near-death stage, and complete mortality, respectively; is essential for formulating irrigation schemes of vegetation grown in compacted soil under drought conditions. The effect of soil types on these critical soil suctions are unexplored and is crucial in understanding the soil-specific plant water functions. This study aims to establish the drought response of Axonopus compressus (grass), based on stomatal conductance (gs) and chlorophyll fluorescence parameters (CI) grown in different soil types. A. compressus were grown in six soil types (2 coarse-grained and 4 fine-grained soils) for 8 weeks, followed by continued drought condition. The gs and CI were monitored along with soil suction and moisture content. Both leaf and root growth were observed to be higher in coarse-grained soils than fine-grained soils, even though the water retention of the coarse-grained soils were comparatively less. Drought stress initiation in plants was captured by ψthreshold from the CI (especially in fine-grained soils) before the gs response. The three critical soil suctions estimated from the correlation between CI and ψ were found to be increasing with higher soil clay fraction. Corresponding plant available water contents (based on v/v volumetric water content) with each of three critical soil suctions were found to be dependent on the relative growth of canopy to root growth that occurred in different soil medias. Especially, plant available water in ‘tipping suction’ was dependent on the soil clay fraction (i.e., higher fraction could restrict root water uptake) and is presented with a simple empirical correlation for A. compressus.

Geotechnical characterization of tropical soil via laboratory testing

This manuscript presents the evaluation of relevant geotechnical properties of the soil profile in the northern region of Campinas, which are important for geotechnical engineering practice and development in São Paulo State. Disturbed and undisturbed samples from the subsoil were collected through the excavation of inspection shafts at the experimental site for Soil Mechanics and Foundations research at the State University of Campinas, located at the Faculty of Civil Engineering, Architecture and Urbanism (FEC). Laboratory tests were conducted, including soil characterization, soil shear strength, soil compressibility, soil permeability, and soil suction tests. The characterization tests indicated that the upper soil layer is subject to surface effects and more direct weathering action, with noticeable variations in the liquid and plasticity limit values. The permeability test results showed no significant variation between the values obtained in the vertical and horizontal directions. The soil-water characteristic curves for the studied profile were bimodal, typical of tropical soils with macro and microaggregated structures.

Seismic Response of Pile Foundations in Clayey Soil Deposits Considering Soil Suction Changes Caused by Soil–Atmospheric Interactions

Seismic Response of Pile Foundations in Clayey Soil Deposits Considering Soil Suction Changes Caused by Soil–Atmospheric Interactions

Variability in the measured soil‒water characteristic curve with respect to the numbers of specimens

The soil‒water characteristic curve (SWCC) defines the relationship between the amount of water in soil and soil suction. The SWCC is commonly used to estimate the hydraulic conductivity function (HCF) and the shear strength function (SSF). Therefore, an accurate determination of the SWCC is crucial for implementing the principles of unsaturated soil mechanics. The SWCC is commonly determined from a limited number of experimental data because SWCC measurements are time-consuming and costly. As a result, the minimum number of required soil specimens is crucial for a SWCC test when considering the accuracy of the determined SWCC and the experimental expenses. In this study, both engineered from sand and kaolin mixtures and residual soils from Bukit Timah Formation in Singapore are selected to prepare soil specimens for SWCC measurements. The SWCCs obtained from the specimens with engineered soil are consistent, while those from specimens with residual soil are slightly scattered. This indicates that one specimen is sufficient to determine the SWCC for engineered soil samples, while a minimum of two specimens should be prepared for the determination of SWCC for residual soil samples from Bukit Timah Formation.

Compressibility of expansive soil mixed with sand and its correlation to index properties

Prior research has primarily focused on Atterberg limits, void ratios, and/or water content, often disregarding the impact of coarse material percentage in the soil, which significantly affects compressibility behavior. This paper examines the effects of sand content, initial degree of saturation, and initial dry unit weight on the compressibility behavior of expansive soils. Ninty-six oedometer tests were performed in order to accurately predict the compressibility behavior of expansive soils. The previous studies have attempted to correlate compressibility with different index properties separately, but no single study has taken into consideration all properties influencing compressibility behavior, especially for expansive soils. The findings show that compressibility is greatly influenced by the sand content, initial degree of saturation, and initial dry unit weight. Increasing the initial dry unit weight specifically lowers the compression index and permeability while raising the recompression index for the same percentage of added sand. Moreover, since swelling reduces with increasing initial saturation, raising the saturation degree also lowers the permeability, recompression index, and compression index. The results indicate that a sand content of more than 30 % is recommended for achieving desired properties in expansive clayey soil. This is a result of sand taking the dominant role in the soil mixture, which lowers soil suction and improves soil properties by reducing swelling, permeability, and compressibility. Symbolic regression equations were created to predict the compression and recompression indices, outperforming previous models in accurately predicting the compressibility behavior of expansive soils, considering the percentage of sand. The validation of these equations demonstrates their predictive capabilities.

Water-holding properties and electrochemical characterization of the soil under the Grand Deliverance Hall of the Chongshan Temple, Taiyuan City, China: providing the basis for preventing groundwater erosion in historic buildings

Many historic buildings are at serious risk of deterioration due to changes in the moisture content of the soil under the buildings. Based on the typical deterioration problems of the Grand Deliverance Hall and the cultural relics in the hall, this study analyzes and concludes that the main factor affecting the generation and development of deterioration is groundwater erosion through environmental monitoring and geotechnical engineering investigation. The impact of changes in moisture content on the water-holding properties and pore distribution of the soil under the cultural-relics buildings was further assessed. Then, combined with the soil property, our study provides a novel and effective way to monitor and reflect the water-holding properties of the soil using electrochemical detection. The connected pore resistance and matrix suction decrease with the increase in the moisture content, both showing a power function law. Changes in moisture content cause changes in the thickness of the double electric layer, which in turn affects the connected pore resistance and matrix suction of soil. The greater the connected pore resistance, the greater the matrix suction and the water holding capacity. Finally, based on the research of this study and the conservation ideology of cultural relics that respects the original and minimizes intervention, protection recommendations to prevent groundwater erosion are proposed in order to provide guidance for future research.Graphical

SELECTION OF THE OPTIMAL MACHINERY COMPLEX FOR CONSTRUCTING IRRIGATION CANALS WITH A PARABOLIC PROFILE

An analysis of the experience of dredger operation in various industries allows us to identify the following main problems that need to be addressed to improve their efficiency: improvement of papillonation schemes and structural enhancement of the main equipment from a hydraulic perspective. The current soil intake devices, suction lines, and soil pumps do not always fully meet the specific requirements of soil development conditions. The objectives of the present research were to propose ways to enhance the performance of systems for automatic control of dredger work movements and automatic regulation of the soil intake process. These proposals aim to create a parabolic cross-sectional shape of the canal during development, ensure the highest productivity and lowest energy consumption, as well as low work costs, using modern control devices. The effect of the proposed technology for canal development using dredgers with automated papillonation consists of the following: providing the developed canals with a stable cross-sectional shape; automating the papillonation of the dredger and optimizing the operation of the soil pump; Implementing shift-based production accounting; using soil meters in the automated system. Giving the developed canal a cross-sectional shape that is stable in both hydraulic and static aspects has allowed for the following benefits: increased sediment transport capacity of the flow, while simultaneously reducing the cross-sectional area, growth of vegetation, water level fluctuations, water losses, and the right-of-way strip; reduced the volume of cleaning work by up to 20%; increased the period between cleanings by ensuring uniform flow movement; enhanced the productivity of dredgers by concentrating sediment on the canal slopes without changing the overall volume of cleaning.

Preliminary Experiences in Determining the Soil–Water Characteristic Curve of a Sandy Soil Using Physical Slope Modeling

Relating soil moisture content to soil suction, the soil–water characteristic curve (SWCC) represents an essential feature in unsaturated soil mechanics that enables estimation of different unsaturated soil property functions and modeling of the macro-scale soil behavior. However, depending on the soil and processes under consideration, proper hydraulic characterization of a soil through direct laboratory measurements can be difficult, time-consuming, and involve many uncertainties. In the case of uniformly graded sands, there is a highly nonlinear and steep shape of the SWCC, with only a few kPa of soil suction separating saturated and residual soil moisture conditions, which makes measurements for determinations of SWCC especially challenging. This study encompasses an investigation of the sandy type of soil’s behavior and presents some preliminary results and experiences on the determination of SWCC through the use of physical slope model tests. The 30 cm deep slope, inclined at 35 degrees and instrumented with soil moisture and pore water pressure sensors, was exposed to series of rainfall intensities, ranging from 37 up to 300 mm/h. The results indicated that the data on hydraulic response in monitored points are not only useful for the determination of SWCC, but that the approach is useful for investigation of hydraulic hysteresis phenomena, as well as its effects on soil moisture and pore water pressure conditions, which also affects the stability conditions of a slope. In particular, the best-fit parameters of the van Genuchten model suggested air entry values of 1.6 and 1.1 kPa for the drying and the wetting curves of the SWCC, respectively, with the two branches shifted by about 1 kPa of soil suction.

Unsaturated soil nailing wall system reliability analysis using random finite element

This study focuses on the evaluation of stability for a typical unsaturated Soil Nailing Wall System (SNWS), considering variations in soil shear strength, suction, and elasticity modulus. The Random Finite Element Method (RFEM) is employed to derive the reliability indices for stability modes, including Global Stability (GS), Lateral Displacement Stability (LDS), Tensile Strength Stability (TSS), and Pullout Resistance Stability (PRS), which serve as essential components of the SNWS. Finally, the Sequential Compounding Method (SCM) is utilized to combine these modes of stability and their direct influence on the system reliability index. The results show although the maximum Coefficient of Variation (COV) of the soil parameters was 11%, the COV of lateral displacements at the top of the wall was obtained as 19%, which shows the significant influence of considering the soil heterogeneity. Furthermore, uncertainties of elasticity modulus and matric suction lead to an increase in GS, LDS, and PRS. The calculated system reliability index was found to be lower than the lowest reliability index of the individual components. The findings indicate that while the inclusion of matric suction and elasticity modulus may enhance the mean values of stability modes, there is a possibility that the system reliability index could decrease.

Mechanical and hydraulic properties of unsaturated layered pyroclastic ashes in landslide-prone areas of Campania (Italy)

Air-fall pyroclastic soil deposits usually display a loose fabric composed of alternating layers of ashes and pumices. Such deposits, when lying on steep slopes, represent a major geohazard due to the occurrence of landslides. This is the case of the carbonate massifs in Campania (southern Italy), a wide landslide-prone area of approximately 400 km2 covered with pyroclastic soils. In such cohesionless deposits, the additional shear strength provided by soil suction in unsaturated conditions is important for ensuring slope stability and can be jeopardized by soil wetting during rainwater infiltration. This paper provides a comprehensive view of the hydraulic and shear strength characteristics of different layers of pyroclastic deposits at different sites in Campania, revealing a broad view of their similarities and differences. To that end, some datasets from previous studies and novel data are gathered, linking the index properties, the hydraulic behavior of the soils and the contribution of suction to the shear strength of the studied materials. Two types of ashes at different positions within the stratigraphic sequence are identified: ashes interbedded between pumice layers, where landslide failure surfaces usually occur, and altered ashes in contact with the bedrock, which affects water leakage from the overlying soil profile. The former show quite uniform characteristics, and this allowed testing some predictive models for the assessment of the unsaturated shear strength of pyroclastic ashes in the absence of direct measurements. In contrast, the latter may exhibit significantly different behaviors, with great variability in hydraulic and mechanical properties.

Exploring an eco-friendly approach to improve soil tensile behavior and cracking resistance

Soil tensile strength is a critical parameter governing the initiation and propagation of tensile cracking. This study proposes an eco-friendly approach to improve the tensile behavior and crack resistance of clayey soils. To validate the feasibility and efficacy of the proposed approach, direct tensile tests were employed to determine the tensile strength of the compacted soil with different W-OH treatment concentrations and water contents. Desiccation tests were also performed to evaluate the effectiveness of W-OH treatment in enhancing soil tensile cracking resistance. During this period, the effects of W-OH treatment concentration and water content on tensile properties, soil suction and microstructure were investigated. The tensile tests reveal that W-OH treatment has a significant impact on the tensile strength and failure mode of the soil, which not only effectively enhances the tensile strength and failure displacement, but also changes the brittle failure behavior into a more ductile quasi-brittle failure behavior. The suction measurements and mercury intrusion porosimetry (MIP) tests show that W-OH treatment can slightly reduce soil suction by affecting skeleton structure and increasing macropores. Combined with the microstructural analysis, it becomes evident that the significant improvement in soil tensile behavior through W-OH treatment is mainly attributed to the W-OH gel's ability to provide additional binding force for bridging and encapsulating the soil particles. Moreover, desiccation tests demonstrate that W-OH treatment can significantly reduce or even inhibit the formation of soil tensile cracking. With the increase of W-OH treatment concentration, the surface crack ratio and total crack length are significantly reduced. This study enhances a fundamental understanding of eco-polymer impacts on soil mechanical properties and provides valuable insight into their potential application for improving soil crack resistance.

Coupled effects of temperature and suction on the anisotropic elastic shear moduli of unsaturated soil

Elastic shear moduli of soil at various temperatures and suctions are important for analysing the serviceability limit state of energy piles and many other structures. Up to now, however, the coupled effects of temperature and suction on elastic shear modulus and the stiffness anisotropy, have not been well understood. This experimental study investigated the anisotropic elastic shear modulus of a compacted lateritic clay. A temperature and suction-controlled triaxial apparatus equipped with bender element probes and local strain measurements was used. Soil suctions from 0 to 300 kPa, and a temperature range of 5–40 °C were applied. The results at saturated and unsaturated conditions consistently reveal that the shear modulus is smaller after heating at a given stress and suction. Several mechanisms may contribute to this thermal-induced reduction in shear modulus, such as the heating-induced reduction of interparticle force and air–water surface tension. Moreover, the reduction in shear modulus upon heating depends on the shear plane and the degree of anisotropy changes.

Soil water potential measurement using fiber-optic sensing technique

Accurate measurement of soil water potential or suction is crucial for hydrology and geotechnical engineering. Inspired by recent advances in fiber-optic sensing techniques, we develop an innovative geophysical method for measuring the soil suction by using fiber Bragg grating (FBG), named FBG-suction method. In this study, working principles and theoretical background associated with the FBG-suction method are discussed. For verification purposes, a series of laboratory-scale tests including calibration, suction measurement, and evaporation monitoring are performed on two clayey soils of low to high plasticity under different degrees of saturation. The calibration and measurement results demonstrate the efficacy and accuracy of the developed method for measuring a wide range of soil suction. The curves of soil saturation with suction obtained by WP4C and FBG-suction method are comparable and reflect the difference in the water retention capacity of the tested soils. Additionally, the evaporation monitoring results show that the FBG-suction method can measure the low suction during the drying process and accurately obtain the high suction above the measuring range of the PST-55 psychrometer. The findings affirm the scientific viability of the fiber-optic sensing technique for acquiring the spatial distribution of water potential or suction in unsaturated soils and provide a novel perspective for future investigations into the spatiotemporal evolution characteristics of in-situ unsaturated soil under climate change. As interdisciplinary collaborations continue to advance, it becomes increasingly apparent that methods based on fiber-optic sensing have the potential to revolutionize traditional approaches to characterizing dynamic subsurface processes.

Evaluation of the Impact of Climate Variability on the Soil-Water Characteristics Curve

The hydro-mechanical behavior of unsaturated soil, particularly expansive soil, is influenced significantly by cyclic wetting and drying. Understanding the soil parameters is crucial when evaluating the performance of infrastructures constructed on expansive clay. As a result of extreme rainfall events, highway slopes containing highly expansive Yazoo clay in Mississippi, U.S., become vulnerable to volume change. The phenomenon creates perched water zones within the slopes and poses a risk of slope failure. The soil-water characteristic curve (SWCC) defines the relationship between water content and soil suction, which can be obtained from different laboratory procedures. However, conventional laboratory methods have some limitations. To address this, various analytical and predictive models have been developed, but they can only offer estimates based on soil characteristics and lack seasonal variations occurring in field conditions. Studying seasonal SWCC through field measurements can help understand soil responses to changing moisture conditions. The current study utilized field data from six highway slopes in Mississippi and classified the data into different seasons: spring, summer, and fall. After obtaining van Genuchten parameters from the fitted curve for each season, the finite element method was applied to evaluate the parameters for accurate numerical analysis of infrastructures containing expansive clay. The study observed the variations in flow parameters with seasonal change that cannot be achieved when data from only one season is considered. The findings underscore the importance of field instrumentation data for developing SWCC and the significance of seasonal flow parameters in infrastructure design.

A framework for estimating the matric suction in unsaturated soils using multiple artificial intelligence techniques

AbstractImplementation of the state‐of‐the‐art understanding of the mechanics of unsaturated soils into geotechnical engineering practice is partly limited due to the lack of quick, reliable, and economical techniques for matric suction measurement. Matric suction is one of the key stress state variables that significantly influences the hydro‐mechanical behavior of unsaturated soils. In this paper, to address this objective, two artificial intelligence (AI) models were developed for estimating matric suction in unsaturated soils based on the particle swarm optimization support vector regression (PSO‐SVR) and multivariate adaptive regression spline (MARS) algorithms. The results suggest that both these models can reasonably estimate matric suction. Compared to the MARS model, the PSO‐SVR model can achieve higher accuracy. Nonetheless, the MARS model facilitates the sensitivity analysis and the selection of essential inputs. A novel integrated framework is proposed and validated, leveraging the strengths, and alleviating the limitations of the PSO‐SVR and MARS algorithms for reliable and rapid estimation of matric suction in the range of 0–1500 kPa for low plastic soils (0 < Ip ≤ 7). Six inputs are required to use this model successfully; some can be measured using conventional laboratory tests, and others can be calculated from mass‐volume relationships.

In situ monitoring of the impacts of Melaleuca styphelioides transpiration on soil water dynamics in an urban environment

Expansive soils are susceptible to substantial cyclical moisture variations under the prevailing climate, experiencing considerable shrinkage during drought and swelling when water availability increases. These dynamic volumetric changes jeopardize the functionality of lightweight infrastructures with shallow foundations. The presence of trees may amplify seasonal soil movements, as extensive water uptake by root networks in the summer months can induce soil moisture depletion and the resulting desiccation-driven settlement around the trees. This study assessed the drying effect of mature Melaleuca styphelioides on soil movement and water dynamics at an urban experimental site in Melbourne, Australia, instrumented for in situ monitoring over 12 months. Sap flow instrumentation was used to quantify seasonal variations in tree transpiration. The monitoring results revealed that the drying influence of the tree has extended to a depth of 2.2 m. Tree root-induced soil desiccation occurred 4.0 m away from the tree, as evidenced by the seasonal soil movement profiles. Linear regression analysis showed that the soil water content explained 60% of the variability in tree transpiration. In contrast, the soil water content and suction exhibited a robust negative correlation (R2=0.96) within the active root zone.